Sintered body of silicon nitride and its manufacture

Abstract

A sintered body of silicon nitride with high density, strength, toughness, and hardness, which can be used for a structural part of an engine, etc. The sintered body of the invention includes 80 to 94 wt % Si 3 N 4 , 2 to 10 wt % Mg compound calculated in MgO equivlent, and 2 to 10 wt % Y compound calculated in Y 2 O 3 equivalent. The Si 3 N 4 contains 5 to 40% α-phase. The sintered body is manufactured in the following steps: (a) mixing powdery Si 3 N 4 with a Mg compound and a Y compound in the above proportions, and then molding the mixture in which the Si 3 N 4 powder contains 80% or more α-phase and its grains are 1 μm or less in diameter on average; (b) performing primary sintering at 1,600° C. or less in an atmospher of nitrogen or of an inert gas at 20 atm or less; and (c) performing secondary sintering at 1,400° to 1,600° C. in an atmosphere of nitrogen or of an inert gas at 300 atm or more.

What is claimed is: 
     
       1. A sintered body of silicon nitride having a relative density of 98% or more, comprising 80 to 94 wt % Si 3  N 4 , 2 to 10 wt % compound calculated in MgO equivalent, and 2 to 10 wt % Y compound calculated in Y 2  O 3  equivalent, wherein the Si 3  N 4  contains 5 to 40% α-phase and the Si 3  N 4  comprises grains 90 vol % or more of which have a minor axis of 1 μm or less. 
     
     
       2. A process for manufacture of a sintered body of silicon nitride comprising the steps of: (a) mixing 80 to 94 wt % powdery Si 3  N 4  with 2 to 10 wt % Mg compound calculated in MgO equivalent and with 2 to 10 wt % Y compound calculated in Y 2  O 3  equivalent, and then molding the mixture, in which the Si 3  N 4  powder contains 80% or more α- phase, and the grains are 1 μm or less in diameter on average;   (b) performing primary sintering of the molded object in step (a) at 1,600° C. or less in an atmosphere of nitrogen or nitrogen and an inert gas at 1 to 20 atm to provide a sintered body having a relative density of 85% or more; and   (c) performing secondary sintering of the sintered body in step (b) at 1,400° to 1,600° C. in an atmosphere of nitrogen and an inert gas at 300 atm or more, such that 90 vol % or more of the grains in the sintered body produced have a minor axis of 1 μm or less.     
     
     
       3. A process for manufacture of a sintered body of silicon nitride as in claim 2 wherein said secondary sintering is conducted in an atmosphere of nitrogen and an inert gas, in which the nitrogen partial pressure in both the primary and secondary sintering is 1 atm or more. 
     
     
       4. A process for manufacture of a sintered body of silicon nitride as in claim 2 in which the temperature in the secondary sintering is in the range of 1,450° to 1,550° C.